LIBRARY

SYNOPSIS

DESCRIPTION

The file name specified by path is opened for either execution or reading
and/or writing as specified by the argument flags and the file descriptor
returned to the calling process. The flags argument may indicate the
file is to be created if it does not exist (by specifying the O_CREAT
flag). In this case open() and openat() require an additional argument
mode_tmode, and the file is created with mode mode as described in
chmod(2) and modified by the process’ umask value (see umask(2)).
The openat() function is equivalent to the open() function except in the
case where the path specifies a relative path. In this case the file to
be opened is determined relative to the directory associated with the
file descriptor fd instead of the current working directory. The flag
parameter and the optional fourth parameter correspond exactly to the
parameters of open(). If openat() is passed the special value AT_FDCWD
in the fd parameter, the current working directory is used and the
behavior is identical to a call to open().
The flags specified are formed by or’ing the following values
O_RDONLY open for reading only
O_WRONLY open for writing only
O_RDWR open for reading and writing
O_EXEC open for execute only
O_NONBLOCK do not block on open
O_APPEND append on each write
O_CREAT create file if it does not exist
O_TRUNC truncate size to 0
O_EXCL error if create and file exists
O_SHLOCK atomically obtain a shared lock
O_EXLOCK atomically obtain an exclusive lock
O_DIRECT eliminate or reduce cache effects
O_FSYNC synchronous writes
O_SYNC synchronous writes
O_NOFOLLOW do not follow symlinks
O_NOCTTY don’t assign controlling terminal
O_TTY_INIT restore default terminal attributes
Opening a file with O_APPEND set causes each write on the file to be
appended to the end. If O_TRUNC is specified and the file exists, the
file is truncated to zero length. If O_EXCL is set with O_CREAT and the
file already exists, open() returns an error. This may be used to
implement a simple exclusive access locking mechanism. If O_EXCL is set
and the last component of the pathname is a symbolic link, open() will
fail even if the symbolic link points to a non-existent name. If the
O_NONBLOCK flag is specified and the open() system call would result in
the process being blocked for some reason (e.g., waiting for carrier on a
dialup line), open() returns immediately. The descriptor remains in non-
blocking mode for subsequent operations.
If O_FSYNC is used in the mask, all writes will immediately be written to
disk, the kernel will not cache written data and all writes on the
descriptor will not return until the data to be written completes.
O_SYNC is a synonym for O_FSYNC required by POSIX.
If O_NOFOLLOW is used in the mask and the target file passed to open() is
a symbolic link then the open() will fail.
When opening a file, a lock with flock(2) semantics can be obtained by
setting O_SHLOCK for a shared lock, or O_EXLOCK for an exclusive lock.
If creating a file with O_CREAT, the request for the lock will never fail
(provided that the underlying file system supports locking).
O_DIRECT may be used to minimize or eliminate the cache effects of
reading and writing. The system will attempt to avoid caching the data
you read or write. If it cannot avoid caching the data, it will minimize
the impact the data has on the cache. Use of this flag can drastically
reduce performance if not used with care.
O_NOCTTY may be used to ensure the OS does not assign this file as the
controlling terminal when it opens a tty device. This is the default on
FreeBSD, but is present for POSIX compatibility. The open() system call
will not assign controlling terminals on FreeBSD.
O_TTY_INIT may be used to ensure the OS restores the terminal attributes
when initially opening a TTY. This is the default on FreeBSD, but is
present for POSIX compatibility. The initial call to open() on a TTY
will always restore default terminal attributes on FreeBSD.
If successful, open() returns a non-negative integer, termed a file
descriptor. It returns -1 on failure. The file pointer used to mark the
current position within the file is set to the beginning of the file.
When a new file is created it is given the group of the directory which
contains it.
The new descriptor is set to remain open across execve(2) system calls;
see close(2) and fcntl(2).
The system imposes a limit on the number of file descriptors open
simultaneously by one process. The getdtablesize(2) system call returns
the current system limit.

RETURNVALUES

If successful, open() and openat() return a non-negative integer, termed
a file descriptor. They return -1 on failure, and set errno to indicate
the error.

ERRORS

The named file is opened unless:
[ENOTDIR] A component of the path prefix is not a directory.
[ENAMETOOLONG] A component of a pathname exceeded 255 characters, or
an entire path name exceeded 1023 characters.
[ENOENT] O_CREAT is not set and the named file does not exist.
[ENOENT] A component of the path name that must exist does not
exist.
[EACCES] Search permission is denied for a component of the
path prefix.
[EACCES] The required permissions (for reading and/or writing)
are denied for the given flags.
[EACCES] O_TRUNC is specified and write permission is denied.
[EACCES] O_CREAT is specified, the file does not exist, and the
directory in which it is to be created does not permit
writing.
[EPERM] O_CREAT is specified, the file does not exist, and the
directory in which it is to be created has its
immutable flag set, see the chflags(2) manual page for
more information.
[EPERM] The named file has its immutable flag set and the file
is to be modified.
[EPERM] The named file has its append-only flag set, the file
is to be modified, and O_TRUNC is specified or
O_APPEND is not specified.
[ELOOP] Too many symbolic links were encountered in
translating the pathname.
[EISDIR] The named file is a directory, and the arguments
specify it is to be modified.
[EROFS] The named file resides on a read-only file system, and
the file is to be modified.
[EROFS] O_CREAT is specified and the named file would reside
on a read-only file system.
[EMFILE] The process has already reached its limit for open
file descriptors.
[ENFILE] The system file table is full.
[EMLINK] O_NOFOLLOW was specified and the target is a symbolic
link.
[ENXIO] The named file is a character special or block special
file, and the device associated with this special file
does not exist.
[ENXIO] O_NONBLOCK is set, the named file is a fifo, O_WRONLY
is set, and no process has the file open for reading.
[EINTR] The open() operation was interrupted by a signal.
[EOPNOTSUPP] O_SHLOCK or O_EXLOCK is specified but the underlying
file system does not support locking.
[EOPNOTSUPP] The named file is a special file mounted through a
file system that does not support access to it (e.g.
NFS).
[EWOULDBLOCK] O_NONBLOCK and one of O_SHLOCK or O_EXLOCK is
specified and the file is locked.
[ENOSPC] O_CREAT is specified, the file does not exist, and the
directory in which the entry for the new file is being
placed cannot be extended because there is no space
left on the file system containing the directory.
[ENOSPC] O_CREAT is specified, the file does not exist, and
there are no free inodes on the file system on which
the file is being created.
[EDQUOT] O_CREAT is specified, the file does not exist, and the
directory in which the entry for the new file is being
placed cannot be extended because the user’s quota of
disk blocks on the file system containing the
directory has been exhausted.
[EDQUOT] O_CREAT is specified, the file does not exist, and the
user’s quota of inodes on the file system on which the
file is being created has been exhausted.
[EIO] An I/O error occurred while making the directory entry
or allocating the inode for O_CREAT.
[ETXTBSY] The file is a pure procedure (shared text) file that
is being executed and the open() system call requests
write access.
[EFAULT] The path argument points outside the process’s
allocated address space.
[EEXIST] O_CREAT and O_EXCL were specified and the file exists.
[EOPNOTSUPP] An attempt was made to open a socket (not currently
implemented).
[EINVAL] An attempt was made to open a descriptor with an
illegal combination of O_RDONLY, O_WRONLY, O_RDWR and
O_EXEC.
[EBADF] The path argument does not specify an absolute path
and the fd argument is neither AT_FDCWD nor a valid
file descriptor open for searching.
[ENOTDIR] The path argument is not an absolute path and fd is
neither AT_FDCWD nor a file descriptor associated with
a directory.

SEEALSO

HISTORY

The open() function appeared in Version 6 AT&T UNIX. The openat()
function was introduced in FreeBSD 8.0.

BUGS

The Open Group Extended API Set 2 specification requires that the test
for whether fd is searchable is based on whether fd is open for
searching, not whether the underlying directory currently permits
searches. The present implementation of the openat checks the current
permissions of directory instead.